The Early History of Reentry Physics Research at Lincoln Laboratory J.

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The Early History of Reentry Physics Research at Lincoln Laboratory J.
The Early History of
Reentry Physics Research
at Lincoln Laboratory
Leo J. Sullivan
IIIDuring the period from June 1958 through September 1965 Lincoln
Laboratory engaged in a reentry measurements program that developed radar
and optical instrumentation for observing the phenomena associated with the
hypervelocity reentry of payloads into the earth's atmosphere. This article
describes both the Lincoln Laboratory effort and the development and
launching of reentry test vehicles by NASA.
1953 THE SOVIET UNION exploded their first thermonuclear device, and the possibility of their joining that device with a long-range ballistic missile
became a real threat for the United States [1]. Government and research facilities realized that this threat called
fOr an increased research effort to study reentry phenomena. Discussions in 1958 with representatives ofthe then
National Advisory Committee for Aeronautics (NACA),
located at Langley Air Force Base, Va., revealed that
NACA was designing a test vehicle to reenter small payloads into the earth's atmosphere at velocities in the
neighborhood of 20,000 fi:lsec. At the same time, Lincoln Laboratory, motivated by the threat posed by the
Soviet Union's successful development of long-range
ballistic missiles, became interested in the problem of
warhead reentry and in the radar and optical phenomena
associated with hypersonic reentry into the earth's
The importance ofcontrolled experiments with known
aerodynamic configurations to study teentry phenomena became obvious, and Lincoln Laboratory joined forces with NACA in a cooperative Reentry Physics Research
Program. NACA, which became the National Aeronautics and Space Administration (NASA) on 1 Oerober
1958, was responsible for the design and launch of reentry vehicles, and Lincoln Laboratory was responsible for
the development and construction ofthe radar and optical instrumentation to observe the reentries, along with
the gathering and interpretation of data.
Foundations in Meteor Research
In 1958 the knowledge of the phenomena associated
with the hypersonic entry of a body into the earth's
atmosphere had been obtained only by observing natural
meteors. By using optical and radar techniques, meteor
astronomers had accumulated considerable knowledge
of the effeers produced by meteors interacting with the
earth's atmosphere. The earliest radar observations of
meteors had been made at long wavelengths (approximately 30 MHz) by using both pulsed and continuouswave transmissions. At that frequency only the meteor
trail could be seen. Later, because of increased power
generation capability at higher frequencies, the signals
reflected from the ionized region in the immediate vicinity of the meteor were observed at 200 MHz and above.
The production of an ionized trail by meteors was
well known, and the degree of ionization in the trail
(whether it was underdense or overdense) was well understood. At frequencies of 200 MHz and greater, socalled head echoes, or radar reflections from the ionized
plasma in front of the reentry body, were observed. The
head echoes traveled at the same speed as the meteor, and
the combination ofthe Doppler frequency and the rangeversus-time data of the head echo provided a direct
measure ofmeteor velocity.
Results of meteor research led to the following con-
VOLUME 4, NUMBER 2, 1991
The Early History ofReentry Physics Research at Lincoln Laboratory
clusions: (1) radar returns are obtained from the ionized
trail, or wake, produced by a reentry body, (2) radar
backscattering from the ionized bow shock, or region in
the immediate vicinity in front of the reentry body, is
observed, and (3) the critical frequencies for radar measurements ofa reentry body are found in the microwave
region between UHF (400 MHz) and X-band (10 GHz).
Radar Instrumentation
On the basis ofthe results ofmeteor research we decided
that radar observations of a reentry body should be
made at three different frequencies throughout the complete trajectory. The frequencies selected were UHF (400
MHz), S-band (3 GHz), and X-band (10 GHz). We
further decided that the rocket vehicle should be automatically tracked in angle and range by radar, starting
immediately afrer launch and going through all stage
separations, including reentry, without the aid of any
radiating device such as a beacon or transponder in the
vehicle. We selected the S-band frequency of 3 GHz as
the operating frequency for the angle-tracking system.
The UHF and X-band frequencies, along with the
S-band, were used to measure the cross sections of the
bow shock and turbulent wake of the reentry body.
The reentry body was a 5-in sphere with an S-band
cross section of0.01 m . Tracking a reentry body of this
size out to a range of 200 nmi imposed a demanding
requirement on the tracking radar. This tracking requirement, along with limited component availability,
essentially determined the design of the S-band angletracking radar.
Because of time pressures on the program, we decided to use an existing 60-fr parabolic antenna mounted
on a surplus u.s. Navy Dual 5-in/38-caliber gun mount
(see Figure 1). The most powerful S-band transmitter
available at that time was the transmitter used with the
Air Force FPS-6 height-finding radar. The magnetron
in this transmitter developed a peak output of approximately 4.5 MW at a pulse width of 2 flSec,
which represented a good compromise between range
FIGURE 1. Radar antennas at the Lincoln Laboratory field site at Arbuckle Neck, Va., located near the NASA Wallops
Island launch facility. Site instrumentation consists of (from right to left) the S-band tracking radar, the multiplexed
UHF and X-band cross-section measurements radar, and the SPAN DAR long-range trajectory and range-safety
radar designed by Lincoln Laboratory for NASA.
VOLUME 4. NUMBER 2. 1991
The Early History ofReentry Physics Research at Lincoln Laboratory
resolution and sensitivity.
In spite of the advantages of four-horn monopulse
for angle uacking and signal-amplitude measurement,
conical-scan angle tracking was selected because a lownoise receiver was needed to insure a signal-to-noise
ratio adequate for uacking the small target at maximum
range. The state of the art in receiver design at the time
was such that low-noise amplification using a maser or
parametric amplifier was feasible only in a conical scan
receiver that required only a single channel. The phase
and amplitude stability of three separate RF channels
with parameuic amplification, which was required by a
monopulse system, had not been demonstrated at that
time. Accordingly, we decided to use conical-scan angle
tracking with a niuogen-cooled parametric amplifier
that was designed and built at Lincoln Laboratory. Later, when the phase stability of cooled parametric amplifiers had progressed sufficiently, the target angle-tracking subsystem was modified to a four-horn amplitude
monopulse configuration.
We decided that the UHF and X-band radars should
be diplexed in a second available 60-ft antenna that was
also mounted on a Navy 5-in/38-caliber gun mount;
this diplexed radar was then slaved to the S-band tracker
(Figure 1). Lincoln Laboratory had previously developed high-powered UHF transmitters and modulators
in suppon of the Boston Hill search radar, which was a
prototype for the Air Force FPS-35 search radar. The
VA 812e series UHF klystron, which was capable of a
power output of approximately 8 MW at the pulse
widths and pulse repetition rates ofinterest, was selected
as the transmitter tube for the UHF radar, and a transmitter unit and modulator essentially identical with the
Boston Hill system was procured. The scattering cross
sections of the ionized wake and flow field were not well
known at the time. We knew that the cross sections were
low, but at the same time we recognized the advantages
ofrange resolution. The alternating uansmission ofpulses
that were 6 f.1Sec and 1 f.1Sec in duration was chosen to
give both maximum pulse energy (to measure the cross
sections) and accurate range resolution.
The UHF transmitter consisted ofa power amplifier
chain excited by a stable continuous-wave source, but
the limited availability ofresources initially led to the use
of simple amplitude detection, which did not take advantage of the coherent capability inherent in the UHF
transmitter system. The UHF receiver system was later
modified by the addition ofa coherent signal processing
and recording system. A square-horn UHF feed allowed
both venical and horiwntal polarization. Transmission
was at the vertical polarization, with simultaneous reception of both the vertically and horiwntally polarized
backscattered components. The UHF feed illuminated
the full 60-ft apenure of the antenna.
The X-band cross-section measurement system utilized the same reflector as the UHF cross-section measurement system. Because of the narrow beamwidth
produced by a fully illuminated 60-ft reflector at
X-band, the rectangular X-band horn located in the
center of the UHF horn was restricted to illuminating
only an elliptical area approximately 20 ft X 15 ft. By
illuminating a smaller area of the antenna we increased
the width of the X-band beam, which kept the target
body more easily within the beam. We utilized the highest-power X-band transmitting tube available at that
time, which consisted ofa magnetron with a peak power
of approximately 1 MW at a pulse width of 2 f.1Sec.
A low-noise front-end preamplifier was essential to
maximize the sensitivity of the receiver. At the time,
because of the lack of a parametric X-band preamplifier, a maser was the only choice as a low-noise preamplifier. Accordingly, Lincoln Laboratory designed and
built the world's first X-band maser preamplifier and
installed it in the X-band radar at Arbuckle Neck, Va.
Installation ofRadar Instrumentation
The NASA head start in designing and building a launch
vehicle forced the radar instrumentation program at Lincoln Laboratory into a tail chase from the beginning.
Trailblazer Ia was launched by NASA on 3 March 1959
and Trailblazer Ip was launched on 4 June 1959. Both of
these launches occurred without any radar coverage at all
and with optical insuumentation consisting only of surplus K-2 aerial ballistic cameras with rotating choppers
to provide a time base. No spectral coverage existed at
the time.
The S-band tracking radar built at Lincoln Laboratory and tested at Millstone Hill became operational at Arbuckle Neck in time for the 1 December 1959 launch of
the Trailblazer Iy vehicle from Wallops Island. The
S-band radar acquired the rocket vehicle after launch,
during the early up-firing stage burning, and switched
The Early History ofReentry Physics Research at Lincoln Laboratory
2). This radar used a 60-ft paraboloidal antenna with a
conical-scan feed identical to that on the S-band tracking
radar. It was mounted on a Millstone Hill radar antenna
mount instead of a surplus Navy gun mount like those
used for the other Lincoln Laboratory radars. Even though
the transmitting and receiving systems ofthe SPANDAR
radar resembled those of the S-band tracking radar, its
design and operating parameters were intended for longrange tracking ofsatellites and rocket vehicles in both the
multiple-pulse beacon transmitter mode and the skintracking mode.
Radar Data Collection and Processing
FIGURE 2. The 6O-ft antenna and mount of the SPAN DAR
S-band tracking radar designed by Lincoln Laboratory
for NASA.
track at each stage separation, including the 5-in spherical rocket-body sixth stage. The radar continued to track
throughout reentry. The installation of the UHF and
X-band measurement systems at Arbuckle Neck soon
followed the launch ofTrailblazer Iy, and the completely integrated multiwavelength measurement system
became operational.
Concurrently with the design and construction of the
initial S-band tracking radar and the X-band and UHF
radar mentioned above, Lincoln Laboratory developed a
second high-power S-band tracking radar, the Space Range
Radar (SPANDAR), for NASA and installed it at the
Lincoln Laboratory field site at Arbuckle Neck (Figure
VOLUME 4. NUMBER 2. 1991
The Arbuckle Neck radar system was one of the first
systems to be specifically designed as an integrated multiple-frequency cross-section measurement system. Figure 3 is a photograph of the integrated measurement
system as it looked after installation in 1959. From the
beginning of the program the guiding principle was that
the radar complex should be designed as a complete
data-gathering and processing system with the data gathering performed at Arbuckle Neck (which was on the
Vtrginia mainland west of the Wallops Island launch
site) and all data processing and analysis performed at
Lincoln Laboratory in Lexingron, Mass.
Direct digital recording on magnetic tape was selected
as the primary method for recording all radar signals.
The signals at the three radar frequencies (and for both
polarizations of the UHF radar) were simultaneously
time-gated with range gates, and the peak value reached
during the gating interval was digitized and recorded on
magnetic tape. The time, azimuth, elevation, and range
already existed in digital form and were recorded simultaneously with the signal data. The original digital system recorded each parameter on separate tape tracks.
This integrated digital measurement system was the
first highly capable digital data-recording system in the
After a launch, the data tape was transported to Lexingron, where it was read into the IBM 704 at the central
computer facility. During the period of the reentry program, the 704 was successively superseded by the 709,
the 7090, and finally the 7094. Because the data tape was
not in IBM format, translation equipment-the so-called
Codal translator-was required to play back the digital
data into the real time input ofthe 704 and its successors.
The Early History ofReentry Physics Research at Lincoln Laboratory
FIGURE 3. The integrated multiple-frequency cross-section measurement and recording system. This system was
the first highly capable digital data-recording system in the country.
Because the digital system measured and recorded
signal amplitude at only one range station, it was quickly
supplemented by oscilloscope film recording that was
designed to record over a range interval extending in
front of and behind the tracked target. Film traces of
multiple or extended targets could be examined visually
for qualitative analysis, and trace deflections could be
read manually to obtain preliminary quantitative results.
Manual film reading was a slow and laborious process,
however, so an automatic film reader was designed and
constructed. This reader consisted of a precision smallscreen cathode ray tube whose spot focused on the moving film and scanned in a direction perpendicular to the
recorded trace under the command of a PDP-l digital
computer. The displacements of the scanned spot were
digitized and recorded as a measure of the amplitude
versus time of the analog video signal originally recorded
by the oscilloscope.
Optical Measurements
The production ofoptical radiation by meteors entering
the earth's atmosphere was well known, and wide fieldof-view optical measurement techniques and instrumentation to record meteor activity had reached a highly
developed state. Meteor cameras had undergone an evolution from the early f/6.3 meteor patrol cameras built in
the thirties to the highly sensitive Super Schmidt cameras
designed by James Baker and built in the 1940s by the
Perkin-Elmer Corporation. The Super Schmidt cameras
had a corrected field ofview ofapproximately 55° and a
geometric f number of 0.84, which made them extremely fast and useful for meteor photography. Spectrographic measurements were made by placing a dispersing
element, such as a prism or grating, over the aperture.
Although astronomers had photographed thousands of
natural meteor trails, they had recorded only a few meteor spectra up to 1932, primarily because of the lower
sensitivity due to the dispersion of the spectrographic
camera. Interest in meteor spectra substantially increased
afrer 1932, because of the increased performance of
camera equipment and the development of higherspeed films, and by 1958 approximately 300 spectra
were available.
In 1958 four Super Schmidt meteor trail cameras
were in the possession of Harvard Observatory, which, in
conjunction with the Smithsonian Astrophysical Observatory in Cambridge, Mass., was the world's leading
center for meteor research. Because of this fact we decided that cooperation with Harvard Observatory would be
fruitful, so we initiated a contract for the installation of
two of Harvard's Super Schmidt cameras at the Lincoln
Laboratory field site at Arbuckle Neck and the assistance
VOLUME 4, NUMBER 2, 1991
The Early History ofReentry Physics Research at Lincoln Laboratory
FIGURE 4. Optical instrumentation at the Lincoln Laboratory field site at Arbuckle Neck.
At the left is one of the Harvard Observatory Super Schrnidt cameras; in the middle is an
identical Super Schmidt camera converted to a spectrograph by a triple-element Fresnel
prism mounted in front of the camera aperture. The smaller Baby Schmidt camera at the
far right, equipped with a grating, functions as a slitless spectrograph. The operator is
Lawrence Prugnarola.
of Harvard Observatory staff in fUm processing, film
reading, and data reduction. One of the Super Schmidt
cameras was turned into a spectrograph by mounting a
three-element plastic prism in front of the camera aperture. Both Super Schmidt cameras were equatorially
mounted and were driven at the sidereal rate.
Because the performance of the Super Schmidt spectrograph was poor in the ultraviolet region, it was supplemented by the so-called Baby Schmidt camera, which
was designed and built by the Perkin-Elmer Corporation
to Lincoln Laboratory specifications. This smaller f/0.83
Schmidt camera had a 6-in aperture and a 20 0 field of
view. Its corrector plate was fabricated from sagged Vicor
glass whose optical transmission extended below 3000 A.
Another small Schmidt camera owned by Harvard Observatory, the Paul Schmidt (named after the amateur
astronomer who built it), was used with film sensitive in
the near infrared to extend the wavelength coverage into
that region. The Baby Schmidt camera and the Paul
VOLUME 4, NUMBER 2, 1991
Schmidt camera, because of their small field of view
compared to the azimuth dispersion ofthe reentry event,
were mounted on a single servo-driven two-axis altitudeazimuth mount slaved to the S-band tracking radar. The
two cameras operated in track mode on this mount until
shordy before the time of the reentry event; at that time
the mount position was frozen and the cameras operated in trail mode. Figure 4 shows the two Super Schmidt
cameras, along with the Baby Schmidt and the Paul
Schmidt cameras, installed at the Arbuckle Neck site.
Simultaneous optical observations from two separated stations are required to determine the path and velocity of a reentry object. A second Super Schmidt camera
station was therefore located at Eastville, Va., southwest
of the Wallops Island launch site. This second camera
position provided the required second observation of the
reentry event. An ideal site for the second camera station
was actually located farther south along the North Carolina coast; this ideal second site had a vantage point that
The Early History ofReentry Physics Research at Lincoln Laboratory
Wallops Island 37"50' N 75°29' W
Honest John Impacts at 7 sec
Nike Impacts at 182 sec
5-in Spherical Rocket
Visible at 352.6 sec
at 230,000 ft
T-55 Rocket Visible
at 371.2 sec
at 230,000 ft
T -40 Impacts at 436 sec
FIGURE 5. Map of the Delmarva Peninsula and the Virginia and North Carolina coastline regions, and the offshore
impact regions of the Trailblazer launch vehicles. The NASA launch facility was located at Wallops Island, and the
Lincoln Laboratory field site was located on the mainland at Arbuckle Neck, Va. A second camera position at
Eastville, Va., provided the necessary additional optical measurements of the reentering object.
could have provided minimwn range, high elevation
angle, and long baseline. The early beginnings of the
Wallops Island program, however, took place before construction was completed on the bridge-tunnel connecting the southern tip of the Delmarva peninsula with
Virginia and North Carolina, thus making travel to an
ideal coastline observation point difficult. The travel
distance and the shortage of manpower led to the decision to confine our optical measurement activities initially to the Delmarva Peninsula near Wallops Island, even
though it was measurably poorer for optical observations. Accordingly, the second Super Schmidt camera
station was located at Eastville, Va. Additional cameras
were later installed at Coquina Beach, N.C. Figure 5
VOLUME 4, NUMBER 2, 1991
The Early History ofReentry Physics Research at Lincoln Laboratory
FIGURE 6. The 1.2-m Cassegrain telescope with the dual-wavelength spectrometer mounted on top. The spectrometer subdivided and measured the
energy in the infrared region from 0.6 to 4.0 ,um and in the visible region
from 0.3 to 0.6 ,um. The operator is James Daley.
illustrates the geography of the area around the Wallops
Island launch site, along with the second camera locations and the impact regions of the launch vehicles.
Tracking Spectrometer
Early in the program we recognized 'the limitations placed
on the optical observations by the low sensitivity of the
wide field-of-view meteor cameras. We decided to design
and build a narrow field-of-view tracking spectrometer
that could either be slaved to the S-band tracking radar
or operated autonomously by automatically angle-tracking the self-luminous plasma immediately adjacent to
the reentry body.
A 1.2-m-apenure Cassegrain telescope configuration
was selected for the light collector. This telescope consist-
VOLUME 4. NUMBER 2, 1991
ed of an f/5 paraboloidal primary mirror and'a 16-indiameter hyperboloidal secondary mirror that was mounted on a tip/tilt mechanism to provide the wideband
vernier tracking loop required to track the high angular
rates and accelerations of the reentry object. The overall
f number of this telescope was f/15. The telescope was
mounted on a high-precision azimuth/elevation mount
powered by gearless torque motor drives; inductosyns
were used for angular position measurement ofthe mount.
The analog angle outputs were digitized and used for
data recording and transmission.
The converging beam reflected by the secondary mirror passed through a hole in the primary mirror toward a
focus at the angle-error sensor that operated in the visible
spectral region. A beam splitter placed before the focus of
The Early History ofReentry Physics Research at Lincoln Laboratory
the telescope reflected part of the received energy to a
lithiwn-fluoride prism. The prism separated the energy
into two spectral bands-the visible from 0.3 to 0.6 J1II1
and the infrared from 0.6 to 4.0 ,um-and dispersed and
imaged the infrared energy on 10 contiguous PbS detectors whose outputs were digitally recorded on magnetic
tape. The visible energy was directed into a CzernyTurner grating spectrometer equipped with 30 photoncounting photomultipliers that were cooled by circulating liquid freon. The outputs of the individual
photomultipliers were then counted, cyclically scanned,
digitized, and recorded on magnetic tape. Figure
6 shows the 1.2-m Cassegrain telescope with the spectrometer mounted above it.
The spectrometric telescope system was completed in
1963 near the end of the reentry measurements program. It was the most higWy developed system ofits kind
at the time, and it successfully recorded visible spectrometric data on the last launch of the program.
The Development of the
Trailblazer I Launch Vehicle
The reentry test vehicle designed by NASA was designated Trailblazer I. Figure 7 shows a Trailblazer I vehicle
on the launcher at the NASA Wallops Island launch
facility. The vehicle consisted of a six-stage solid-propellant rocket configuration in which the first three stagesthe Honest John, Nike, and XM45 rockets-were successively fired to loft the velocity package, consisting of a
nose shell containing three down-firing stages, to an
altitude of approximately 1 million ft, or 200 mi. The
down-firing motors were the T40, the T55, and the 5-in
spherical motor. Figure 8 illustrates the configuration of
the Trailblazer I launch vehicle and the velocity package,
and Figure 9 shows a typical trajectory for a Trailblazer I
The last down-firing stage reached a velocity of approximately 20,000 ftlsec, which is necessary to simulate
realistically the phenomena generated by the reentry of
an ICBM payload. This velocity requirement, along with
the impulse limitations ofeach of the down-firing stages,
imposed an upper limit of two pounds on the weight of
the last stage, which in turn required that the last-stage
rocket motor casing become the reentry body at motor
burnout, and also required that the total impulse-tomotor-casing weight ratio be maximized.
FIGURE 7. The six-stage Trailblazer I reentry vehicle on
the launcher at the NASA Wallops Island launch facility.
The entire launch vehicle, which was approximately 55 ft
long, launched the velocity package to a height of approximately 200 mi. The three-stage down-firing velocity
package reached a final velocity of 20,000 ft/sec.
A spherical motor, which is the optimwn configuration for withstanding the high internal pressures
generated by the burning propellant, was chosen as
the sixth-stage, or reentry-stage, motor for the Trailblazer
I rocket vehicles. To generate and radiate a continuous-wave signal at the 200-MHz frequency needed to
measure Doppler velocity, NASA mounted a cylindri-
VOLUME 4. NUMBER 2. 1991
The Early History ofReentry Physics Research at Lincoln Laboratory
_ _. ......_--XM45
N;k, ----..'
Altitude Stages - - - - - - - - - - - -....~~I
5-in Spherical Rocket Motor
Radar Beacon
Velocity Package
FIGURE 8. The overall configuration of the Trailblazer I launch vehicle and the velocity package containing the three
down-firing stages. (a) The first three stages were Honest John, Nike, and XM45 rockets, and (b) the second three
stages, the down-firing stages, were T40, T55, and 5-in spherical rockets.
cal metallic housing containing a transistorized transmitter on the front of the spherical-rocket-motor reentry body. The cylinder was insulated from the sphere
and formed one half of a dipole that, along with the
sphere, radiated in a toroidal radiation pattern. Figure 10
shows the configuration of the Trailblazer I reentry
body with the cylindrical housing mounted on the front
of the spherical rocket motor. The first two Trailblazer
launches, designated Trailblazer Ia and Trailblazer I~,
used this reentry configuration. The signal from the
reentry-body transmitter provided a measure of Doppler velocity. We soon recognized, however, that the
blunt end of the cylinder (which was in contact
with the hypersonic shock wave and was immediately
followed by the ionized stagnation region) would severely influence the radar backscattering characteristics
of those regions and would also substantially complicate the interpretation of the data on the backscat122
VOLUME 4. NUMBER 2. 1991
5-in Sphere
I - Nike
I - Honest John
Range (ft)
FIGURE 9. The staging sequence and trajectory for the
Trailblazer I vehicle.
The Early History ofReentry Physics Research at Lincoln Laboratory
FIGURE 10. Early Trailblazer I reentry-body configuration
with a continuous-wave transmitter and antenna mounted
on front of the 5-in spherical motor. This configuration
was used on the first two Trailblazer I launches.
tering from the ionized wake.
The arrival and installation of the Lincoln Laboratory
S-band tracking radar eliminated the need for the NASAinstalled continuous-wave beacon on the reentry body.
Deleting the beacon placed the spherical reentry body
in contact with the shock wave; this change simplified our measurements because the hypersonic flow characteristics of a spherical reentry body were better
understood and described by aerodynamic theory at
that time. The continuous-wave telemetry signal was
then radiated by a dipole antenna in which the original
cylindrical housing was replaced by an annular metallic cylinder containing the transmitter components.
The annular cylinder was attached to the aft end of
the reentry body, where it surrounded the nozzle. An
insulating ring electrically isolated it from the motor
body, with which it formed a dipole radiator. Figure 11
shows the modified configuration for the Trailblazer 1
reentry body; succeeding Trailblazer 1 reentry payloads
beginning with Trailblazer Iy and Trailblazer la through
Trailblazer Ij had this configuration. Table 1 summarizes the detailed characteristics of the Trailblazer
launch vehicles along with their firing dates and reentrybody weights.
Development of the Trailblazer II Launch Vehicle
While experiments utilizing the Trailblazer 1 launch vehicles progressed, we realized that the two-pound maxi-
mum payload weight necessary to achieve a 20,000 fr/sec
reentry velocity imposed a severe limitation on the configuration of the reentry object, which prevented a realistic simulation of many real-world scenarios. We decided
accordingly that NASA would design a new four-stage
rocket vehicle-designated Trailblazer II-with the capability of propelling a 35-pound reentry object to a
velocity of20,000 fi:lsec. Figure 12 is a photograph ofthe
Trailblazer II launch vehicle, and Figure 13 illustrates the
configuration of the launch vehicle and the reentry stage.
This new vehicle allowed more realistic reentry payload
configurations and also allowed the installation of telemetry sensors in the reentry body. Figure 14 shows a
typical Trailblazer II trajectory. Trailblazer II firings commenced in 1963 shortly after the completion of the
Trailblazer 1 series oflaunches.
Summary of Trailblazer Reentry Results
At the start of the Trailblazer program, little was known
about the electromagnetic scattering created by the ionized shock layer and turbulent wake generated by a
hypervelocity vehicle reentering the atmosphere. The
strong radar signals observed at S-band and UHF in the
early Trailblazer reentries, even when viewed at an angle
broadside to the wake, led to the hypothesis that the
FIGURE 11. Later Trailblazer I reentry-body configuration
with the annular transmitter housing and antenna mounted
aft of the spherical motor, surrounding the motor nozzle.
Deleting the transmitter from the original configuration
in front of the spherical reentry body placed the body
directly in contact with the shock wave.
VOLUME 4, NUMBER 2, 1991
The Early History ofReentry Physics Research at Lincoln Laboratory
Table 1. Trailblazer I Launch Vehicle Characteristics
Model No.
Date Fired
Reentering Configuration
3 March 1959
Thin-Wall Aluminum Case
with Spike Telemeter
4 June 1959
Thin-Wall Aluminum Case
with Spike Telemeter
1 December 1959
Heavy-Wall Titanium Case
with Torus Telemeter
29 March 1960
Heavy-Wall Steel Case
with Torus Telemeter
26 June 1960
Heavy-Wall Aluminum Case
with Torus Telemeter
28 August 1960
Heavy-Wall Titanium Case
with Torus Telemeter
29 August 1960
Aluminum Case with Phenolic
Nylon, Torus Telemeter
21 October 1960
Heavy-Wall Steel Case
with Torus Telemeter
17 January 1961
Aluminum Case with Copper
Shield, Torus Telemeter
21 April 1961
7th-Stage 2-g Steel Pellet in
Aluminum Case
18 May 1961
Steel "J" Case with
Dummy Torus Telemeter
16 September 1961
Aluminum Case (Thick Wall)
with Dummy Torus Telemeter
2 April 1962
Aluminum Case, Ablating Phenolic
Nylon, Dummy Torus Telemeter
27 July 1962
8-in Aluminum Case
major scattered signals came from a turbulent wake rather
than any laminar shock layer or near-wake region. The
Trailblazer I reentry vehicle was essentially a sphere, and
this shape removed questions about effects due to angle
of attack and aspect angle. The range extension of the
returned radar pulses demonstrated that the scattering
target indeed was extended in space behind the spherical
reentry body. A statistical analysis ofthe amplitude of the
returns from the trailing portions of the time-stretched
pulses showed that the signals were essentially Rayleigh124
Reentry Weight (lbs)
VOLUME 4, NUMBER 2, 1991
distributed. This analysis demonstrated that the signal
was scattered from an ensemble of multiple scatterers, as
would be expected in scattering from an ionized turbulent region.
Further evidence of the random or noisy character of
the signal scattered from the wake was found in the first
attempts to measure the deceleration of the reentry body
more precisely by utilizing the phase information obtained from the coherent UHF radar. Although the low
repetition rate of the UHF radar resulted in velocity
The Early History ofReentry Physics Research at Lincoln Laboratory
ambiguities, these ambiguities could be resolved and the
phase progression in the signal from the single hard
target could be followed until scattering from the wake
was observed. At the point when the wake was observed,
the phase information became noisy, and the velocity
ambiguities could no longer be resolved because of the
low repetition rate of the radar.
Evidence that the major portion of the energy scattered from the wake came from low-velocity regions was
first obtained by the Millstone Hill Radar (lo<i:ated in
Westford, Mass.) observations of Trailblazer I reentries.
The Millstone Hill Radar had poor range resolution but
good Doppler resolution, and the data showed that most
of the scattered energy during reentry appeared near zero
Doppler. Thus we concluded that the major portion of
the wake was traveling at low velocities. The pulse width
of the UHF radar at Arbuckle Neck was then shortened
to 0.25 )1sec to increase the range resolution, and the
repetition rate was increased to 960 pulses per second to
reduce the ambiguity problem. The data recorded following this modification helped to confirm the earlier
evidence of low velocities in the wake.
The first observations of high-altitude initial shocklayer formation and of the cross-section dip at or nearinitial shock formation were made in the Trailblazer I
experiments. Figure 15 shows the UHF cross section
versus altitude for a Trailblazer I launch on 27 July 1962;
the dip in cross section clearly occurs at a height of
approximately 250,000 ft. Because the reentry bodies
were spherical, no variation in cross section resulted from
changes in the aspect angle; with a more complex object
the changes in aspect angle would mask the effects of
plasma formation. The Trailblazer I experiments with
spherical reentry bodies, such as the object illustrated in
Figure 16, strongly influenced the similar Bell Laboratory experiments in the early 1960s. In these experiments
spheres were flown as secondary payloads along with
reentry measurement vehicles launched from Vandenberg Air Force Base located in California into the Kiernan Reentry Measurements Site (KREMS) located on
Roi-Namur Island in the Marshall Islands Group in the
western Pacific Ocean.
The study of the radar pulse shapes prior to the
appearance of the large wake return frequently revealed
the appearance of apparent wake instabilities that increased in amplitude and eventually produced the large
FIGURE 12. Trailblazer II four-stage reentry vehicle on the
launcher at the NASA Wallops Island launch facility.
wake return. A detailed pulse-by-pulse examination of
the formation of the wake return showed that the wake
return first appeared at some distance behind the reentry
body, increased in magnitude, and moved toward the
reentry body as it descended in altitude. Measurements
of the distance from the body to the beginning of the
wake return at high altitude on some of the reentries
showed close agreement with the prediction of transition
distance from scaled hypervelocity ballistic-range measurements on subscale reentry objects.
VOLUME 4. NUMBER 2. 1991
The Early History ofReentry Physics Research at Lincoln Laboratory
Velod', Package \
Sepa,a"on Dev;,e
1st-Stage Configuration (in)
3rd-Stage Configuration (in)
FIGURE 13. Trailblazer II reentry vehicle configuration. (a) The two up-firing stages of the launch vehicle consisted of
Castor XM-33 and Lance TX-77 rockets; (b) the two down-firing stages consisted of an X-248 rocket and a reentry
payload propelled by a 15-in spherical rocket motor.
Examination of the cross-polarized returns at UHF
(i.e., the polarization orthogonal to the transmitted linear polarization) showed that a cross-polarized component started to appear shortly after initial wake
onset. The orthogonal return was associated with
the peak of the normal wake return and continued to
grow until at the time of the peak wake return the
orthogonal component was nearly equal to the normal
Artificial Meteors
For many years meteor astronomers had a strong interest
in the determination of the total meteoric mass intercepted by the earth per unit time. The brighmess and
velocity of a natural meteor could be measured byexisting chopped-track meteor cameras. The mass remained
unknown, however, and the luminosity coefficient that
VOLUME 4, NUMBER 2. 1991
relates the deceleration and brighmess of a meteor to its
mass had yet to be determined. The creation and measurement of an artificial meteor of known mass was the
only solution.
Trailblazer I reentry experiments routinely achieved
last-stage reentry velocities of approximately 20,000
ft/sec. An additional down-firing stage capable ofadding
a velocity increment of 12,000 ft/sec to a pellet of approximately 2 g would place the reentry velocity of the
pellet in the lower velocity range of natural meteors.
Lincoln Laboratory joined Harvard Observatory, the Air
Force Cambridge Research Laboratories (AFCRL), and
NASA in an experiment to reenter a metallic pellet
weighing approximately 2 g into the earth's atmosphere
at a velocity of32,000 ft/sec by using an explosive accelerator mounted on the front of a Trailblazer I sixth-stage
5-in spherical rocket motor. The explosive accelerator
The Early History ofReentry Physics Research at Lincoln Laboratory
Trailblazer lid
Launch Angle 80°
350 sec
400 sec
360 sec
15-in Spherical
Rocket Ignition 370.2 sec
Spring Release 377.2 sec
380 sec
5-in Retro Ignition 382.2 sec
385 sec
390 sec
395 sec
390 sec
400 sec
410 sec
400 sec
420 sec
Horizontal Range (ft)
FIGURE 14. Typical Trailblazer II four-stage trajectories.
was developed by the University of Utah under contract
to AFCRL. Figure 17 illustrates the explosive accelerator
and the Trailblazer I sixth stage. On 21 April 1961, a
Trailblazer I launch vehicle successfully reentered a 2-g
pellet at a velocity 002,000 ft/sec. The optical data from
this experiment were used to make the first determination of the luminosity coefficient of natural meteors.
Because of the success of this experiment, we planned
another artificial-meteor experiment using the larger Trailblazer II vehicle. In this second experiment, a 5-in spherical motor with an accelerator attached to its ftont surface was in turn mounted on the front of the 15-in
fourth-stage rocket motor on a Trailblazer II launch
vehicle. Figure 18 illustrates this launch vehicle configuration. Mounting the pellet accelerator on the front end
of the 15-in reentry motor of the Trailblazer II increased
the resulting pellet velocity by 15,000 ft/sec to approximately 47,000 ft/sec.
After the Lincoln Laboratory reentry measurements
program terminated in 1965, NASA continued an ex-
panded artificial-meteor program at the Wallops Island
launch facility. The small spectrographic Baby Schmidt
camera was transferred by Lincoln Laboratory to NASA
for use in that ongoing program.
Later Programs
During the course of the operation of the S-band tracking radar, starting in December 1959, returns were observed that were unrelated to any man-made target.
These returns, which were assumed to be from natural
objects, fell into two classes. The first class of returns
exhibited negligible range extent and were usually amplitude modulated; the source of these returns were unknown at first but were later attributed to individual
insects. The second class of returns were due to backscattering from index-of-refraction discontinuities in the atmosphere. This view was reinforced by the occasional
appearance ofhoriwntallayering in the returns. Because
of the time pressures of the Reentry Physics Research
Program, however, both the insect returns and the re-
VOLUME 4. NUMBER 2. 1991
The Early History ofReentry Physics Research at Lincoln Laboratory
Height (1000 ft)
FIGURE 15. The UHF radar cross section versus altitude for the Trailblazer I spherical
reentry body launched on 27 July 1962. This figure shows the typical cross-section dip
at 250,000 ft. This dip occurs immediately prior to the large increase in cross section
caused by the ionized wake behind the reentry body.
turns due to atmospheric discontinuities were not investigated further at that time.
Insect Returns
When the Lincoln Laboratory Reentry Physics Research
Program was terminated in 1965, the tracking and measurement radars at Arbuckle Neck were transferred to
NASA and the U.S. Air Force, and the facility became
known as the Joint Air Force-NASA multiwavelength
radar facility. The radars were then used to carry out a
comprehensive series ofmeasurements on the radar cross
section of individual insects, and the amplitude and
frequency of the cross-section variations due to their
wing motion [2]. These experiments, performed in the
summer of 1965, were a cooperative effort by AFCRL
(which is now the Air Force Geophysics Laboratory), the
Applied Physics Laboratory of Johns Hopkins University, and the Entomology Research Division of the
Department ofAgriculture.
In these experiments single insects in individual con-
VOLUME 4, NUMBER 2, 1991
tainers were carried aloft in a small aircraft that flew at an
altitude above 1.5 km along a radial vector from the
radar position (the vector coincided with the local wind
direction). The S-band radar automatically angle-tracked
the aircraft out to a range of approximately 10 km, at
which point a single insect was released into the slipstream of the aircraft, auto tracking of the aircraft was
discontinued, and the radar line ofsight was frozen. After
the return from the aircraft was sufficiently separated
from that of the insect, automatic angle tracking was
initiated on the insect. Hawkmoths, tobacco budworm
moths, honeybees, and dragonflies were the subjects of
these experiments. The S-band radar cross section of the
hawkmoth, the dragonfly, and the honeybee was typical2
ly 10-3 cm . Measurements were also made at X-band,
but no returns were observed with the UHF radar.
Radar observations of insects provided an answer to
the mystery of dot angels, or echoes observed from otherwise invisible targets in the apparently clear atmosphere.
Extensive research on dot angels showed that they were
The Early History ofReentry Physics Research at Lincoln Laboratory
indeed caused by insects in the air. These experiments
also demonstrated that radar could be successfully used
to measure entomologically significant parameters that
had been considered essentially unmeasurable.
AFCRL exploited the ability of these radars to obtain
returns from atmospheric index of refraction variations
in a major investigation of clear-air turbulence [3]. The
radar observations were made simultaneously at all three
frequencies, namely 400 MHz, 3 GHz, and 10 GHz, by
scanning the antennas slowly in elevation while the video
returns for each wavelength were displayed on separate
range-height indicators whose screens were photographed
for off~line analysis. These scans were performed at azimuths close to those for which the probe aircraft reported an encounter with significant turbulence. The probe
aircraft were the F-lOO, F-4, F-84, F-86, and T-33. An
RB-57 was also included because of its ability to operate
at higher altitudes (up to 50,000 ft), and a C-130 was
included because the effects ofturbulence on that aircraft
closely resembled the effects of turbulence on a commercial aircraft. Both the RB-57 and C-130 were able to
Atmospheric Variations in Index ofRefraction
During the period of the reentry measurements program, we observed returns due to backscanering from
discontinuities in the index of refraction in the atmosphere. These observations were made possible by the
availability of the first high-powered S-band automatic
tracking radar in which high performance was provided
by the high sensitivity and low noise of the first operational liquid-Nrcooled parametric amplifier, the high
transrniner power of the radar, and the 60-ft antenna
From 1967 to 1971 the Weather Radar Branch of
Aluminum Cannon Ball
Magnesium Nozzle
4.0-in Radius
2.5-in Radius
Iliiiiiiiiiiiii~=----+--- Glass
·I· :.
........ -- ......
La min ate
Carbon Nozzle Insert
, , ------
........................ ... _I-
FIGURE 16. Configuration of the 8-in Trailblazer I spherical reentry body launched on 27 July 1962. The
advantage of a spherical reentry body is that radar returns are independent of aspect angle, which
makes the determination of cross section much easier.
VOLUME 4, NUMBER 2, 1991
The Early History ofReentry Physics Research at Lincoln Laboratory
5.056-in Spherical
Aluminum Rocket Motor
Stainless Steel Pellet
25-g Booster Charge
D1i~5~~;er J
Blasting Cap
2-to-4-1 b{ft3
Foam Plastic
140-g PBX Charge
Aluminum Container
4.618 in
(--------10.00 in - - - - - -...
~~f.ooIe----------7.393 in --------1~
FIGURE 17. Sixth and seventh stages of the Trailblazer I vehicle used on 21 April 1961 to reenter the first artificial
meteor at a velocity of 32,000 ft{sec. The seventh stage accelerated a 2-g pellet to a velocity close to that of natural
make longer atmospheric exploration flights. Meteorological data were also taken frequently during the radar
measurements and aircraft flights.
Significant clear-air turbulence was observed during
this program, and the usefulness of a powerful groundbased radar in the detection of clear-air turbulence was
demonstrated. Today, building on the early research
efforts at Wallops Island, the FAA has installed
ground-based radars around the country to provide
information and warning on clear-air turbulence to both
commercial and military aircraft.
Daniel Dustin in the early 1950s recognized the nature
of the ICBM threat, and along with Glen F. Pippen
played a major role in the inception of the Reentry
Physics Research Program at Lincoln Laboratory. This
program, which was sponsored and supponed by the
Defense Advanced Research Projects Agency, ultimately
involved many technical disciplines and crossed many
organizational boundaries. Its success was due to the
effons and cooperation of a large number of dedicated
15-in Spherical Motor
Spin Rockets
5-in Spherical
3rd-Stage Configuration (in)
FIGURE 18. Third, fourth, and fifth stages of the Trailblazer II vehicle used to reenter an artificial
meteor at approximately 47,000 ft{sec. This object combines the reentry stages illustrated in
Figure 17 with the 15-in spherical motor of the Trailblazer II vehicle.
VOLUME 4. NUMBER 2. 1991
The Early History ofReentry Physics Research at Lincoln Laboratory
individuals whose contributions are gratefully acknowledged in alphabetical order as follows: Francisco L.
Bacchialoni, Richard H. Baker, Edward Barsack, John R
Bauer, John Benvento, Edgar Blaisdell, Carl R Bohne,
Arthur E. Buckley, Ralph Burgess, James A. Daley, Crista
de Ridder, Arthur C. Dyer III, Seymour Edelberg, Gabriel
Farrell, Ovid V. Fortier, Claude L. Gillaspie, Eino O.
Gronroos, Victor Guethlen, Paul Harris, John Howard,
Frank Hutchinson, John Hutzenlaub, Kent R Johnson,
Harold L. Kasnirz, Robert H. Kingston, Kent Kresa,
Kenneth E. Leathers, Vito N. Leone, James Marapoti,
Robert Martinson, J.J. Mikulski, RE. McMahon, Robert Meyer, John Paddleford, Edward Peters, Richard
Peterson, Robert Pike, Lawrence J. Prugnarola, Hans W
Rudolph, James R Sandison, George M. Shannon, Julius
J. Sobolewski, Lawrence W Swezey, Edward F. Tarbox,
David Turck, John R. Williamson, Leo Wilber, PA.
Willmann, Frances A. Wilson, and Malcolm WoronofE
In addition to the Lincoln Laboratory personnel list-
ed above, the following NASA personnel played
major roles in the implementation of the Reentry
Physics Research Program: Clarence Brown, R.T.
DuffY, William N. Gardner, WR Hook, Reginald
R Lundstrom, and Ira W Ramsey. Richard McCroskey,
a member of the Harvard Observatory staff, was in
charge of the Harvard effort centered on the Super
Schmidt cameras.
1. LA. Getting, All In a Lifttime-Science in the Defense ofDemocrat)' (Vantage Press, New York, 1989).
2. K.M. Glover, K.R. Hardy, T.G. Konrad, W.N. Sullivan, and
A.S. Michaels, "Radar Observations ofInsecrs in Free Flight,"
Science, 154,967 (25 Nov. 1966).
3. R.]. Boucher, "Evaluation of Clear Air Turbulence Detection
by Ground-Based Radars, Special Rawinsondes, and Aircraft,
1967-1971," Air Force Cambridge Research Laboratories,
Hanscom AFB, MA, 1 Oct. 1974.
VOLUME 4, NUMBER 2. 1991
The Early History ofRl!I!ntry Physics Research at Lincoln Laboratory
is an associate group leader of
the Laser Radar Measurements
Group. He received an S.B.
degree in physics from MIT in
1940. From 1942 to 1945 he
was a Staff member at the MIT
Radiation Laboratory, where he
worked on the development of
automatic angle-tracking radar.
This work led to the development of the SCR-584 gunlaying radar used by U.S. and
British heavy antiaircrafr
batteries during the war. As a
rechnical observer for the War
Department, he received
overseas assignments in
England, North Africa, Iraly,
and France. In 1946 he joined
the L.H. Terpening Co. in New
York and designed radar
components and systems. He
joined the Johns Hopkins
Universiry Applied Physics
Laboratory in 1948 and
became Leader of the Tracking
and Guidance Group that
developed target-tracking and
missile-guidance radars for the
Terrier and Talos missiles. In
1953 he joined Lincoln
Laboratory as a staff member in
the Radar Division and worked
on the development of pulsed
Doppler radar. He later moved
to the Radio Physics Division,
where he designed an
automatic tracking capability
for the Millstone Hill
microwave radar and its RF
feed subsystem. In 1958 he
began working in the area of
reentry physics and its
supporting microwave and
optical instrumentation,
including the design and
construction of the mulriwavelength radars and the 1.2-m
telescope installed at the
Lincoln Laboratory field site
at Arbuckle Neck, Va. He later
participated in the design of
the Project PRESS (Pacific
Range Electromagnetic
Signature Studies) reentry
measurements system at the
Kiernan Reentry Measurements Site (KREMS), located
on Roi-Namur Island in the
Marshall Islands group in the
western Pacific. In 1967 he
joined the Optics Division
when it was formed and
beCame involved in the laser
radar effort at its inception.
He continues to work in that
area. In October 1990 he was
presented with the first IRIS
Active Systems Science and
Technology Award for his
lifetime contributions to the
field of active systems.
VOLUME 4, NUMBER 2, 1991
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